High frequency amplifiers for use in portable electronic devices are required to have relatively high efficiency to satisfy constraints as to size, weight, and power consumption. Radiotelephones and other two-way portable communications devices, termed herein generally as mobile stations, represent a particularly challenging environment for power amplifiers. This is because in mobile stations, power amplifiers are required to provide a substantially linear output over a wide range of radiofrequency RF input power. For example, the code division, multiple access (CDMA) wireless communication standard requires that a power amplifier exhibit good linearity. In the CDMA system, a load mismatch between the power amplifier and the antenna can result in signal distortion and degradation in performance. The end result is that under a load mismatch condition the stringent CDMA transmitter requirements can become more difficult or even impossible to meet. A mismatch might occur due to a change in antenna impedance, such as changing from the ideal 50-ohm free space impedance (which is matched at the power amplifier) to a much higher impedance when a user touches the antenna.
FIG. 1 is a simplified block diagram of relevant portions of a prior art mobile station transmitter 20. A digital signal from a RF source 22 is input into and amplified by a power amplifier PA 24. The power level of the signal from the RF source 22 is controlled by a processor 26 via an input power control 28, which is a control signal and not the RF signal itself. The amount by which the PA 24 amplifies its input signal is determined by a collector voltage VCC supply 30, which allows a particular voltage from a battery 32 or other power source through to the PA 24. The PA 24 outputs the digital signal toward a transmit antenna (not shown). The power of the signal that is output from the PA 24 is sensed at a power detector 34, commonly within a directional coupler. The processor 26 also controls the amount of collector voltage through a VCC control line 38.
There are two instances where power adjustments are made to the circuit of FIG. 1: changes to achieve an established desired output power, and changes to the desired output power itself. Toward the former, the power detector 34 feeds back a measured power output signal 36 to the processor 26. When the processor 26 determines that the measured output power 36 from the power detector 34 varies from a desired level (e.g., the difference between measured and desired power exceeds some threshold), the processor 26 makes an adjustment to the RF source 22 via the input power control 28. For a change to the desired power output itself (e.g., when a transmit antenna experiences a change in impedance that necessitates a new desired PA output power), the processor adjusts the VCC supply 30 via the VCC control 38. For each desired output power from the PA 24, there is one collector voltage.
Prior approaches use a fixed collector voltage for each desired level of PA output power to ensure linearity of output power over all conditions. This is not to say that output power is fixed to a single value, but that for each output power from the power amplifier, there is only one collector voltage. For example, in a CDMA cellular system using power control between a base station and mobile stations within its cell, a particular mobile station may be required to make a power adjustment so that its transmissions will not interfere with those of other handsets. Alternatively, a mobile station user may touch the antenna with his/her hand, increasing load on the antenna. Each of these will result in a change to the desired power output from the power amplifier, and consequently a change in the collector voltage. However, once the new desired output power is determined, there is only one collector voltage matched to that output power. This is shown graphically in prior art FIG. 2, illustrating a one-to-one correspondence between desired output power and VCC. The relation between desired output power and VCC, as well as a relation between input power control 28 and measured/desired output power, is generally stored in a memory 40 that the processor 26 accesses to make adjustments via the input power control 28 and the VCC control 38. The overarching purpose of this approach is directed toward maintaining a linear power output. Various specifications, such as those described above for wireless telecommunications, dictate this linearity.
What is needed in the art is a power amplifier and circuitry to provide linear power outputs that more efficiently use the available limited power supply of a handheld mobile station.